1. Field of the Invention
The present invention relates to a dynamic pulse width modulation (PWM) amplifier, and in particular relates to a dynamic PWM controller.
2. Description of the Related Art
Generally, power amplifiers are divided into four classes: A, B, AB, and C. A class-A amplifier has an output transistor, the operating point of which is in the center of the linear region. The current obtained by the class-A amplifier from a voltage source is constant and does not vary with the level of an input signal. Because the amplitude of an output signal is changed continuously when the class-A amplifier is used to amplify an audio frequency, the class-A amplifier has low efficiency of not more than 25% in practice. Amplification by the class-A amplifier can be performed by a signal-transistor or a pull-push circuit. The class-A amplifier has no crossover distortion and no switching distortion, and the harmonic mainly comprises an even harmonic. The tone quality is thick at a low pitch, smooth at a median, and clear at a high pitch. The class-A amplifier is not popularly used to amplify power due to the large power consumption, low efficiency, easy heating, and greater heat dissipation requirement. Moreover, components within the class-A amplifier have low reliability and short life when they operate with in high current and high temperature over a long period of time, and the class-A amplifier has high cost. Thus, class-A amplifiers are no longer produced. A simple class-A amplifier has only one active component, such as a transistor. Because a bias voltage current is applied to the transistor, it is not completely turned on or off. The non-turned on/off region is referred to as a linear region. An amplifier which operates in the linear region has low distortion, but low efficiency.
A class-B amplifier has a bias voltage circuit, so that pull-push transistors within the class-B amplifier have low current when there is no any driving signal. When a driving signal is provided, the current of one of the two transistors is increased and the other is turned off in the first half period. In the second half period, the operations of the two transistors are exchanged. Because the two transistors operate alternately, a pull-push circuit is required to completely amplify the wave form of signals. The class-B amplifier has high efficiency with efficiency of 78% being the highest in theory. The class-B amplifier, however, has large distortion. The class-B amplifier comprises two transistors which pull and push each other, one outputs current and the other receives current. When a sine wave which is symmetrical with respect to the zero point is to be amplified, one of the transistors is used to amplify the upper half portion of the sine wave, and the other is used to amplify the lower half portion thereof. The two transistors operate alternately to accomplish the amplification operations. The class-B amplifier thus has higher efficiency than a class-A amplifier. However, some undesired problems of the class-B amplifier occu in a non-linear region wherein the sine wave only passes through the zero point. In the non-linear region, one of the two transistors is only just turned on, and the other is only just turned off. Since the transistors require a short transition period to turn on, singles are distorted in the non-linear region.
A class-AB amplifier serves as a class-A amplifier when the level of its driving signal is lower, and it transfers to serve as a class-B amplifier when the level of its driving signal becomes higher. The efficiency of the class-AB amplifier is higher than the class-A amplifier when a small signal serves as an input signal. In the class-AB amplifier, the efficiency is increased as the output power increases. Although there is more distortion in the class-AB amplifier than in the class-A amplifier, the class-AB amplifier is still used, wherein a class-AB of high bias current is popularly used to reduce the distortion of a low level signal. In the configuration of the class-AB amplifier, the class-AB amplifier is composed of a class-A amplifier and a class-B amplifier. The configuration is similar to class-B amplifier, and a circuit which provides small bias current to each transistor is further applied in the class-AB amplifier, so that each transistor is not completely turned off. The class-AB amplifier with high bias current has large power consumption as the class-A amplifier, but it has lesser distortion than class-A amplifier. The class-AB amplifier requires two transistors as the class-B amplifier. The performance of the class-AB amplifier is thus better.
A class-C amplifier has an active component which is turned on in a duration of a small portion of an input signal. The class-C amplifier thus requires a tuned circuit to return the other portions of the input signal by the flywheel effect of the tuned circuit. The class-C amplifier has great distortion, the power of the collector of the class-C amplifier is, however, the greatest. The class-C amplifier is thus used in frequency multiplication circuits or stages of power amplifying. In practice, the oscillation frequency of the class-C amplifier is usually limited by a quartz oscillator with a single output frequency. To raise output frequency, a frequency multiplier is applied to vary the shifting value of the oscillation frequency or FM signal frequency in an integer multiple. The frequency multiplier outputs signal frequency to a non-linear circuit and then obtains required the high-order harmonic from the distorted output wave using a high-multiple resonance frequency set by a resonance circuit. In general, class-C amplifiers with the characteristic of non-linear and resonance circuits operate in coordination to serve as frequency multiplication circuits.